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Unicast Use of the Formerly Reserved 240/4

Document Type Active Internet-Draft (individual)
Authors Seth David Schoen , John IETF Gilmore , David M. Täht
Last updated 2023-07-01
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Internet Engineering Task Force                              S.D. Schoen
Internet-Draft                                                J. Gilmore
Updates: 1122, 3704, 6890 (if approved)                          D. Täht
Intended status: Standards Track         IPv4 Unicast Extensions Project
Expires: 2 January 2024                                      1 July 2023

               Unicast Use of the Formerly Reserved 240/4


   This document redesignates 240/4, the region of the IPv4 address
   space historically known as "Experimental," "Future Use," or "Class
   E" address space, so that this space is no longer reserved.  It asks
   implementers to make addresses in this range fully usable for unicast
   use on the Internet.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 2 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  History of IPv4 Address Types . . . . . . . . . . . . . .   3
     2.2.  Reserved IPv4 Addresses in the RFC Series . . . . . . . .   4
     2.3.  Attempts to Use the "Future Use" Addresses  . . . . . . .   4
     2.4.  Recent Use as Ordinary Unicast Addresses  . . . . . . . .   5
   3.  Change in Status of 240/4 . . . . . . . . . . . . . . . . . .   6
     3.1.  Continued Special Treatment for  . . .   7
   4.  Compatibility and Interoperability  . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     6.1.  Existing Unofficial Uses of 240/4 . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .  15
     A.1.  Operating systems . . . . . . . . . . . . . . . . . . . .  16
     A.2.  Routers and Switches  . . . . . . . . . . . . . . . . . .  17
     A.3.  DHCP implementations  . . . . . . . . . . . . . . . . . .  17
     A.4.  Other implementations . . . . . . . . . . . . . . . . . .  18
     A.5.  Internet of Things  . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   With ever-increasing pressure to conserve IP address space on the
   Internet, it makes sense to consider where relatively minor changes
   can be made to fielded practice to improve numbering efficiency.  One
   such change, proposed by this document, is to redefine the
   "Experimental" or "Future Use" 240/4 region (historically known as
   "Class E" addresses) as ordinary unicast addresses.  These 268
   million IPv4 addresses are already usable for unicast traffic in many
   popular implementations today.  Standardization as unicast addresses
   will eventually allow them to be later deployed by Internet
   stewardship organizations to relieve address space scarcity.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Background

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2.1.  History of IPv4 Address Types

   When the Internet Protocol was being designed, it was unclear whether
   it would be a success, or which of its features might be the key
   features that led to success.  The bulk of its address space was
   dedicated to ordinary "host addresses".  Other blocks and corners of
   the address space were reserved, either for particular protocol
   functions such as loopback, LAN broadcasting, or host bootstrapping,
   or for future definition.  A major allocation of 268 million
   addresses was later made for multicasting [RFC0988], while leaving
   another 268 million reserved for "future use".  After the invention
   of broadcast and multicast, the original ordinary host addresses were
   later described as unicast addresses, which is now the usual

   With decades of hindsight, we can now see that unicast has been the
   success story of the Internet.  Trillions of unicast packets now move
   around the world daily.  By contrast, the non-unicast addresses are
   seldom used.  The use of routable broadcast packets in denial of
   service attacks has now limited broadcast packets to local-area
   networks [RFC2644], and to critical but highly-specialized protocol
   functions such as DHCP [RFC2131], routing updates [RFC1256], or
   neighbor discovery.

   Wide-area multicast packets had a brief research heyday, but never
   reached critical mass.  Today, the overwhelming majority of multiply-
   replicated media streams (such as popular songs and videos,
   television programs, conference calls, and video meetings) are
   carried in unicast packets mediated by application-level replication
   rather than IP-protocol-level multicasting or broadcasting.

   The Internet became a rapid worldwide success.  Partly due to the
   reduction in experimentation that accompanied that success, little
   effort has been paid to looking back at the historical allocations of
   reserved addresses.  The success of unicast traffic has led to a huge
   demand for unicast addresses.  By contrast, there is far more supply
   of reserved, ignored, loopback, and multicast addresses than any
   foreseeable IPv4 Internet will demand.  Most of these historical
   accidents were not carried forward into the IPv6 protocol [RFC4291].
   We propose simple, compatible changes to existing IPv4
   implementations that will increase the supply of unicast addresses by
   redesignating addresses that today are almost completely unused on
   the Internet.  The best and easiest "future use" of many of today's
   formerly reserved IPv4 addresses is as ordinary unicast addresses.

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2.2.  Reserved IPv4 Addresses in the RFC Series

   The Assigned Numbers RFC series reserved various IP addresses or
   assigned them special meanings, starting in 1977 and continuing
   through the early 1990s.  The detailed behavioral requirements for
   IPv4 implementations based on these designations are set out in
   October 1989's RFC 1122 [RFC1122].  As other special cases continued
   to be introduced on occasion, RFC 3232 [RFC3232] announced that IANA
   would track such information in an online database; the present-day
   version of this mechanism is the IPv4 Special-Purpose Address
   Registry [IANA4], as provided for by RFC 6890 [RFC6890].  A wide
   range of host and network software follows these designations by
   treating these Internet addresses specially.

   This document is concerned with the largest special case in RFC 1122:
   the designation of an entire /4 block for Future Use.  In retrospect,
   the flexibility offered by keeping these addresses unused was
   insightful for its time, but since they ended up never being needed
   for any special purposes, they have become the least productive
   portion of the Internet address space.

   The largest block of original addresses reserved for future use in
   1983 was called "Class D" in RFC 870 [RFC0870], and contained what
   would now be called 224/3.  This contained about 536 million
   addresses, about 12.5% of the total available address space.  By
   1986, RFC 988 [RFC0988] split the former Class D in half, designating
   a multicast Class D block, now called 224/4, and a future-use Class E
   block, now called 240/4.  Following the 1993 implementation of CIDR
   [RFC1519] and its 2006 clarification [RFC4632], we no longer speak of
   any IPv4 address as having an "address class," but the reservations
   of these specific addresses that were made by RFC 1122, were
   unaffected by the CIDR change in terminology and routing technology.

2.3.  Attempts to Use the "Future Use" Addresses

   Through the 1980s, there were many reasons to suppose that new forms
   of Internet addressing could emerge, so reserving a substantial
   number of addresses for them was prudent.

   One likely candidate for some time was protocol translation methods
   between IP and other protocols using special surrogate IP addresses.
   This possibility was particularly significant during the time frame
   when IP coexisted widely on heterogeneous networks with other
   protocols.  Special number ranges could have been used to facilitate
   interoperability, protocol translation, or encapsulation between IP
   and non-IP protocols.

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   This prospect received new salience with the adoption of IPv6, where
   some deployed or proposed transition mechanisms use special-purpose
   IPv4 addresses with a distinctive meaning in the context of IPv6
   transition, such as NAT64 [RFC7050] and the deprecated 6to4
   [RFC3068].  While IPv6 transition mechanisms could conceivably have
   used portions of 240/4, they ended up instead using very small
   amounts of special address space from the IETF Protocol Assignments
   block or elsewhere within the unicast space.

   Another form of addressing that was novel in 1989 is anycast
   addressing, in which the same address is used to identify servers at
   physically distinct locations and connected to the Internet at
   different points.  It would have been possible to designate a new
   "class" of addresses for anycast operations.  RFC 1546 [RFC1546],
   which first defined anycast, concluded that this would be a possible
   and even desirable approach:

   |  There appear to be a number of ways to support anycast addresses,
   |  some of which use small pieces of the existing address space,
   |  others of which require that a special class of IP addresses be
   |  assigned.  [...] In the balance it seems wiser to use a separate
   |  class of addresses.

   But anycast services turned out to work fine in most respects by
   using existing unicast routing protocols, existing unicast datagram
   delivery protocols, and ordinary unicast addresses.  They are now
   widely used for specific applications [RFC7094] such as the
   Internet's root nameservers.

2.4.  Recent Use as Ordinary Unicast Addresses

   Overall, 30 years of experience have demonstrated that no new
   addressing mechanism requires the use of 240/4; nor is any likely to
   require it in the future, particularly in light of the IPv6
   transition.  Other explicit reservations such as the IETF Protocol
   Assignments block at have been sufficient.  While it was
   reasonable to plan for an unknown future, the reserved block at 240/4
   did not ultimately aid Internet innovation or functionality.  The
   future has arrived, and it wants IPv4 unicast addresses far more than
   it wants permanently unusable IPv4 addresses.

   The idea of making 240/4 addresses available for unicast addressing
   is not new.  It was suggested by Lear on the influential TCP-IP
   mailing list in 1988 [Lear].  It was formally proposed to IETF more
   than a decade ago, both by Fuller, Lear, and Mayer [FLM], and by
   Wilson, Michaelson, and Huston [WMH].  While the idea of unicast use
   of 240/4 was merely being considered at IETF, the "running code"
   required was simple enough and compatible enough that this behavior

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   change was implemented at that time in several operating systems.
   Then, when the protocol change was ultimately not standardized, those
   implementations remained, but were largely forgotten.  (They are
   summarized in the "Implementation Status" section of this document.)

   The unicast support created in about 2008 in those implementations is
   now running in millions of nodes on the Internet, and has not caused
   any problems over the past decade.  As a result, the 240/4 space has
   been attracting "wildcat" use in private networks; see [VPC].

   Although software support for unicast use of 240/4 is widespread, it
   is not yet universal.  The present document moves this process
   further along by confirming the consensus that unicast is the
   preferred use for 240/4, documenting the exact behavior changes
   required for maximum interoperability, and calling on all vendors and
   implementers to adopt this behavior.  Doing so will prepare for a
   future in which use of these addresses is anticipated and
   unsurprising, so that their allocation can be considered.

   Implementations generally treat public and private addresses
   identically, with the differences occurring only in how routes,
   firewalls, and DNS servers are configured.  The earlier draft [WMH]
   suggested designating the unreserved 240/4 range as [RFC1918]-style
   private address space.  Like the [FLM] draft, this document does not
   attempt to decide or designate whether future allocations from this
   address range will be public or private addresses.  Both options
   require that both hosts and routers be able to use these addresses,
   so the next section fully defines both host and router behavior.

3.  Change in Status of 240/4

   The purpose of this document is to make addresses in the range 240/4
   available for active unicast use on the public Internet.  This
   includes supporting them for numbering and addressing networks and
   hosts, like any other unicast address.

   Host and router software SHOULD treat addresses in the 240/4 range in
   the same way that they would treat other unicast IPv4 addresses.
   Software SHOULD be capable of accepting datagrams from, and
   generating datagrams to, addresses within this range.

   Clients for autoconfiguration mechanisms such as DHCP [RFC2131]
   SHOULD accept a lease or assignment of an address within 240/4
   whenever the underlying operating system is capable of accepting it.

   Other interoperability details related to address-based filtering are
   discussed in a separate section, below.

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3.1.  Continued Special Treatment for

   The address was given a special meaning as a local
   segment limited broadcast address by numerous prior Internet
   standards, starting with RFC 919 [RFC0919] and continuing
   consistently up to the present day.  For example, is
   used as a network-layer destination address in BOOTP [RFC0951] and
   DHCP [RFC2131] for address autoconfiguration broadcasts by hosts that
   don't yet know anything about the networks to which they are
   connected.  While some newer autoconfiguration or autodiscovery
   protocols use other addresses, the use of remains

   The special meaning of was never restricted or
   affected by the reservation of 240/4.  Accordingly, the existing
   distinctive meaning of is unchanged by this document.
   This single address MUST NOT be assigned to an individual host, or
   interpreted as the address of an individual host, even if it would
   otherwise be part of an allocated or announced network block.

4.  Compatibility and Interoperability

   Merely implementing unicast treatment of addresses in 240/4 in
   routers and operating systems, as this document proposes, does not
   cause any compatibility nor interoperability issues.  Hundreds of
   millions of IPv4 nodes currently contain this unicast treatment, and
   all are interoperating successfully with each other and with non-
   updated nodes.

   Compatibility and interoperability issues only arise if and when an
   address from 240/4 is assigned to an interface on a node, and then
   IPv4 packets are exchanged which use such an address as a source or
   destination address.  This document does not recommend doing these
   things, except for testing purposes.

   Older Internet standards counseled implementations in varying ways to
   reject packets from, and not to generate packets to, addresses within

   RFC 1122 [RFC1122], section, states that a "host MUST
   silently discard an incoming datagram containing an IP source address
   that is invalid by the rules of this section."  The same section
   states that Class E addresses are "reserved" (which might be taken,
   in context, to imply that they are "invalid"); the section further
   treats Class A, B, and C as the only possibly relevant address ranges
   for unicast addressing.

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   RFC1812 [RFC1812], section 5.3.7, states that a "router SHOULD NOT
   forward" a packet with such a destination address.  (If section's reference to these addresses as "reserved" is taken to
   imply that they are "special," section 5.3.7 would also imply that a
   "router SHOULD NOT forward" a packet with such a source address.)

   RFC 3704 [RFC3704] (BCP 84) cites RFC 2827 [RFC2827] (BCP 38) in
   asking providers to filter based on source address:

   |  RFC 2827 recommends that ISPs police their customers' traffic by
   |  dropping traffic entering their networks that is coming from a
   |  source address not legitimately in use by the customer network.
   |  The filtering includes but is in no way limited to the traffic
   |  whose source address is a so-called "Martian Address" - an address
   |  that is reserved, including any address within,
   |, or

   In this context, RFC 3704 specifies filtering of these addresses as
   source (not destination) addresses at a network ingress point as a
   countermeasure against forged source addresses, limiting forwarded
   packets' source addresses to only the set which have been actually
   assigned to the customer's network.  The RFC's mention of these
   "Martian Addresses" is based on the assumption that they could never
   be legitimately in use by the customer network.

   Because the 240/4 address space is no longer reserved as a whole, an
   address within this space is no longer inherently a "Martian"
   address.  Both hosts and routers MUST NOT hard-code a policy of
   always rejecting such addresses.  Hosts and routers SHOULD NOT be
   configured to apply Martian address filtering to any packet solely on
   the basis of its reference to a source (or destination) address in
   240/4.  Maintainers of lists of "Martian addresses" MUST NOT
   designate addresses from this range as "Martian".  As noted
   elsewhere, the address retains its special meaning,
   but is also not a "Martian" address.

   The filtering recommended by RFC 3704 is designed for border routers,
   not for hosts.  To the extent that an ISP had allocated an address
   range from within 240/4 to its customer, RFC 3704 would already not
   require packets with those source addresses to be filtered out by the
   ISP's border router.

   Since deployed implementations' willingness to accept 240/4 addresses
   as valid unicast addresses varies, a host to which an address from
   this range has been assigned may also have a varying ability to
   communicate with other hosts.

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   Such a host might be inaccessible by some devices either on its local
   network segment or elsewhere on the Internet, due to a combination of
   host software limitations or reachability limitations in the network.
   IPv4 unicast interoperability with 240/4 can be expected to improve
   over time following the publication of this document.  Before or
   after allocations are eventually made within this range,
   "debogonization" efforts for allocated ranges can improve
   reachability to the whole address block.  Similar efforts have
   already been done by Cloudflare on [Cloudflare], and by RIPE
   Labs on 1/8 [RIPElabs18], 2a10::/12 [RIPElabs2a1012], and 128.0/16
   [RIPElabs128016].  The Internet community can use network probing
   with any of several measurement-oriented platforms to investigate how
   usable these addresses are at any particular point in time, as well
   as to localize medium-to-large-scale routing problems.  (Examples are
   described in [Huston], [NLNOGRing], and [Atlas].)  Any network
   operator to whom such addresses are made available by a future
   allocation will have to examine the situation in detail to determine
   how well its interoperability requirements will be met.

5.  IANA Considerations

   This memo unreserves the address block 240/4.  It therefore requests
   IANA to update the IANA Special-Purpose Address Registry by removing
   the entry for 240/4, whose existing authority is RFC 1122, Section 4.
   Additionally, it requests IANA to update the IANA IPv4 Address Space
   Registry by changing the status of each /8 entry from 240/8 through
   255/8 from "Future Use, 1981-09, RESERVED" to "Unallocated, [Date of
   this RFC], UNALLOCATED".  Finally, IANA is requested to prepare for
   this address space to be addressed in the reverse DNS space in in-

   This memo does not effect a registration, transfer, allocation, or
   authorization for use of these addresses by any specific entity.
   This memo's scope is to require IPv4 software implementations to
   support the ordinary unicast use of addresses in the newly
   unallocated range through  During a
   significant transition period, it would only be prudent for the
   global Internet to use those addresses for experimental purposes such
   as debogonization and testing.  After that transition period, a
   responsible entity such as IETF or IANA could later consider whether,
   how and when to allocate those addresses to entities or to other
   protocol functions such as private addresses.

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6.  Security Considerations

   The change specified by this document could create a period of
   ambiguity about historical and future interpretations of the meaning
   of host and network addresses in 240/4.  Some networks and hosts
   currently discard all IPv4 packets bearing these addresses, pursuant
   to statements in prior standards that packets containing these
   addresses have no agreed-upon meaning.  Such implementations have
   protected themselves from possible incompatible future packet formats
   that might have eventually used these addresses.

   Disparate filtering processes and rules, both at present, and in
   response to the adoption of this document, could make it easier for
   rogue network operators to hijack or spoof portions of this address
   space in order to send malicious traffic.

   Live traffic, accepted and processed by other devices, may
   legitimately originate from these addresses in the future.  Network
   operators, firewalls, and intrusion-detection systems may need to
   take account of this change in various regards, to avoid permitting
   either more or less traffic from such addresses than they expected.

   Automated systems generating reports, and human beings reading those
   reports, SHOULD NOT assume that the use of a 240/4 source address
   indicates spoofing, an attack, or a new incompatible packet format.
   At the same time, they SHOULD NOT assume that the use of 240/4 is
   impossible or will be precluded by other systems' behavior.

   An important concern about the [FLM] and [WMH] drafts was that
   discrepant behavior between systems could create security problems,
   as when a middlebox fails to detect or report an attack or policy
   violation because it believes that an address involved cannot be used
   or cannot be relevant.  Similarly, a logging system could fail to log
   traffic related to 240/4 addresses because it incorporates an
   assumption that no such traffic can ever occur.  Such discrepancies
   between multiple systems' views of communication semantics are a
   common security antipattern.  (Compare [Sherr], exploiting
   discrepancies in telephony equipment's recognition and interpretation
   of DTMF signals.)  Any change to the meaning or status of a group of
   addresses can introduce such a discrepancy.

   In this case, because 240/4 is already commonly supported by several
   widely-used implementations, and is already used for private network
   communications, such discrepancies are already a reality.  If routers
   follow this document's request to cease filtering this address range,
   they will increase the variety of contexts in which implementations
   may receive ordinary unicast packets containing these addresses.
   (Such packets are still unlikely to arrive from distant hosts until

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   some of these addresses are eventually allocated for experimental or
   production use, and until the global routing table receives
   announcements for subnets in this range.)

   The adoption of this document will converge on an explicitly shared
   understanding that implementations should prepare for this
   possibility.  Since unofficial private use of 240/4 addresses is a
   reality today, while any public allocations from this range are still
   distant and contingent on further study, implementers are receiving
   considerable advance notice of this issue.

6.1.  Existing Unofficial Uses of 240/4

   Some organizations are reportedly using portions of 240/4 internally
   as RFC 1918-type private-use address space, for example for internal
   communications within datacenters.  Google has advised hosting
   customers [VPC] that they may use this address space this way.
   RIPE's ATLAS project detected the use of this address space in
   several other large institutions [RIPE240] in 2022.  Future
   allocations of 240/4 could result in use of this space on the public
   Internet in ways that overlap these unofficial private-use addresses,
   creating ambiguity about whether a particular host intended to use
   such an address to refer to a private or public network.  Among other
   unintended outcomes, hosts or firewalls that have extended greater
   trust to other hosts based on their use of a certain unofficial
   network number (that was considered to imply presence on a LAN or
   within an organization) may eventually receive legitimate traffic
   from an external network to which this address space has been

   Operators of networks that are making unofficial uses of portions of
   240/4 may wish to plan to discontinue these uses and renumber their
   internal networks, or to request that IANA formally designate certain
   ranges as additional Private-Use areas.

7.  Acknowledgements

   This document directly builds on prior work by Dave Täht and John
   Gilmore as part of the IPv4 Unicast Extensions Project.

8.  References

8.1.  Normative References

   [IANA4]    Internet Assigned Numbers Authority, "IANA IPv4 Special-
              Purpose Address Registry",

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   [RFC0870]  Reynolds, J. and J. Postel, "Assigned numbers", RFC 870,
              DOI 10.17487/RFC0870, October 1983,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <>.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
              2006, <>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,

8.2.  Informative References

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              Arista Networks, "EOS 4.28.1F User Manual", 18 April 2022,

   [Atlas]    RIPE Network Coordination Centre, "RIPE Atlas",

              Strong, M., "Fixing reachability to, GLOBALLY!", 4
              April 2018, <

   [FLM]      Fuller, V., Lear, E., and D. Meyer, "Reclassifying 240/4
              as usable unicast address space", Work in Progress,
              Internet-Draft, draft-fuller-240space-02, 25 March 2008,

   [Huston]   Huston, G., "Detecting IP Address Filters", 13 January
              2012, <

              Juniper Networks, "Juniper Networks JUNOS 9.6 Software
              Release Notes", 1 June 2010,

              Juniper Networks, "JUNOS OS: Protocol-Independent Routing
              Properties User Guide", 14 March 2022,

   [Lear]     Lear, E., "Re: Running out of Internet addresses?", TCP-IP
              mailing list, 27 November 1988,

              NLNOG RING, "10 Years of NLNOG RING",

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   [RFC0919]  Mogul, J., "Broadcasting Internet Datagrams", STD 5,
              RFC 919, DOI 10.17487/RFC0919, October 1984,

   [RFC0951]  Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
              DOI 10.17487/RFC0951, September 1985,

   [RFC0988]  Deering, S., "Host extensions for IP multicasting",
              RFC 988, DOI 10.17487/RFC0988, July 1986,

   [RFC1256]  Deering, S., Ed., "ICMP Router Discovery Messages",
              RFC 1256, DOI 10.17487/RFC1256, September 1991,

   [RFC1519]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
              Inter-Domain Routing (CIDR): an Address Assignment and
              Aggregation Strategy", RFC 1519, DOI 10.17487/RFC1519,
              September 1993, <>.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, DOI 10.17487/RFC1546,
              November 1993, <>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2644]  Senie, D., "Changing the Default for Directed Broadcasts
              in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
              August 1999, <>.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, DOI 10.17487/RFC3068, June 2001,

   [RFC3232]  Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
              by an On-line Database", RFC 3232, DOI 10.17487/RFC3232,
              January 2002, <>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

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   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,

   [RIPE240]  Lone, Q., "240/4 As Seen by RIPE Atlas", 23 August 2022,

              Aben, E., "The Curious Case of 128.0/16", 6 December 2011,

              Schwarzinger, F., "Pollution in 1/8", 3 February 2010,

              Aben, E., "The Debogonisation of 2a10::/12", 17 January
              2020, <

   [Sherr]    Sherr, M., Cronin, E., Clark, S., and M. Blaze, "Signaling
              vulnerabilities in wiretapping systems", IEEE Security &
              Privacy November-December 2005,

   [VPC]      Google Inc., "Virtual Private Cloud: Subnets overview:
              Valid IPv4 ranges",

   [WMH]      Wilson, P., Michaelson, G., and G. Huston, "Redesignation
              of 240/4 from "Future Use" to "Private Use"", Work in
              Progress, Internet-Draft, draft-wilson-class-e-02, 29
              September 2008, <

Appendix A.  Implementation Status

   The IPv4 protocol update proposed by this document has already been
   implemented in a variety of widely-used software platforms.  In many
   cases, implementers were persuaded of the value of the suggestions
   contained in [FLM] and [WMH].

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   All known TCP/IP implementations either interoperate properly with
   packets with sources or destinations in the 240/4 range, or ignore
   these packets entirely, except FreeBSD and NetBSD, which have support
   for 240/4 for some purposes while blocking it for others.

A.1.  Operating systems

   240/4 has been supported for transmitting and receiving ordinary
   unicast packets in Linux kernels since linux-2.6.25 was released in
   January 2008.  Creating interfaces in the 240/4 range also worked
   fine using the iproute2 api (as used by the "ip" command) in that
   release.  A kernel patch that allows properly configuring interfaces
   in the 240/4 range using the busybox ifconfig command was released in
   linux-4.20 and linux-5.0 in December 2018.

   240/4 as unicast was released in Fedora 9 in May 2008, and in Ubuntu
   8.10 in October 2008.

   240/4 has been supported as ordinary unicast in the Android mobile
   operating system since Android 1.5 Cupcake (April 2009, using linux-

   240/4 has been supported as ordinary unicast in the OpenWRT router OS
   since OpenWRT 8.09 (September 2008, using linux-2.6.26).  A December
   2018 kernel patch that allows properly configuring interfaces in the
   240/4 range using the ifconfig command was merged into OpenWRT 19.01,
   along with two other patches to netifd and BCP38 that improve support
   for 240/4.

   240/4 has been supported as ordinary unicast in Apple's macOS
   (formerly OS X) operating system and iOS mobile operating system
   since about 2008.

   240/4 has been supported as ordinary unicast in Sun's Solaris
   operating system since about 2008.

   240/4 traffic has been partly supported in OpenBSD for many years and
   is substantially fully supported since OpenBSD 7.2 (released October
   20, 2022).

   240/4 traffic is partly supported for local interface assignment in
   the FreeBSD operating system.  However, ICMP and packet forwarding
   are not supported by default.  Full support for 240/4 addresses has
   been implemented in FreeBSD-current since July 13, 2022.  This
   behavior is disabled by default, but enabled by "sysctl
   net.inet.ip.allow_net240=1".  As of July 2023, this support has not
   yet appeared in a numbered release of FreeBSD; it is scheduled to be
   included in FreeBSD 14.0 in October 2023.

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   We have prepared a patch which enables 240/4 support this on NetBSD.
   It has not been merged as of July 2023.

   240/4 traffic is blocked by default in all versions of the Microsoft
   Windows operating system.  Windows will not assign an interface
   address in this range, if one is offered by DHCP.

A.2.  Routers and Switches

   240/4 has been tested to interoperate as ordinary unicast in 2019 in
   a Cisco router using IOS release, which was also released
   in 2019.  Older and newer releases are also likely to work.

   240/4 traffic is blocked by default in Juniper's router operating
   system, but can be enabled with a simple configuration switch,
   starting from the JUNOS 9.6 release in June 2010.  See page 50 of
   [JUNOS-release-notes-9.6].  It notes, "The JUNOS Software now allows
   Class E addresses to be configured on interfaces.  To allow Class E
   addresses to be configured on interfaces, remove the Class E prefix
   from the list of martian addresses by including the [edit routing-
   options martians 240/4 orlonger allow] configuration statement."  See
   also chapter 5, "Martian Addresses" on page 129 through 136 of the
   2022 documentation [JUNOS-routing-properties].  It includes a
   completely worked example on "Removing the Class E Prefix on Martian

   Arista switches running EOS 4.25.2F (from February 2021), and later
   releases, include the command "ipv4 routable" which
   enables the use of 240/4 addresses on interfaces and in packet
   routing.  The default is to disable this ability.

   The Belkin AX3200 router (with firmware 1.0.01 build 101415 Oct 14,
   2020) cannot use addresses from 240/4 locally, but is happy to route
   packets to such addresses elsewhere in the Internet.

A.3.  DHCP implementations

   Support for 240/4 addressing may be typical of many DHCP
   implementations (because the 240/4 address assignment special case
   has often been handled at the kernel level).  If the underlying
   operating system supports 240/4 assignment to an interface, the final
   official ISC DHCP release (4.4.3) supports 240/4 allocation as both
   client and server, as do Busybox DHCP udhcpc/udhcpd (release 1.1.15),
   and ISC Kea (which currently includes only a DHCP server

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A.4.  Other implementations

   Routing of subnets in the 240/4 range is fully supported by the Babel
   routing protocol and by its main implementation, as of 2020 (or

   Routing of subnets in the 240/4 range is supported by the Gobgp
   routing daemon, as of release 3.0.0 in 2022-03 (or earlier).

   Routing of subnets in the 240/4 range is supported by the BIRD
   routing daemon, as of release 2.0.10 in 2022-06.

A.5.  Internet of Things

   Popular embedded Internet-of-Things environments such as RIOT and
   FreeRTOS already support 240/4 as unicast.

Authors' Addresses

   Seth David Schoen
   IPv4 Unicast Extensions Project
   San Francisco, CA
   United States of America

   John Gilmore
   IPv4 Unicast Extensions Project
   PO Box 170640-rfc
   San Francisco, CA 94117-0640
   United States of America

   David M. Täht
   IPv4 Unicast Extensions Project
   Half Moon Bay, CA
   United States of America

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